1. Field of the Invention
The present invention relates to an electroluminescent device driving circuit used in exposure systems of matrix type electroluminescent display devices and electronic type printing apparatuses. In particular, the present invention relates to a circuit structure of an electroluminescent device driving circuit using amorphous silicon (a-Si) as the semiconductor layer of a film transistor for driving an electroluminescent device.
2. Description of the Related Art
FIG. 6 shows an electroluminescent device driving circuit for one bit of a matrix type electroluminescent display device or electroluminescent device array. The electroluminescent device circuit comprises a first switching device Q1, a storage capacitor Cs whose one terminal is connected to the source terminal of the first switching device Q1, a second switching device Q2 whose gate terminal is connected to the source terminal of the first switching device Q1 and whose source terminal is connected to the other terminal of the storage capacitor Cs, and an electroluminescent device CEL whose one terminal is connected to the drain terminal of the second switching device Q2 and whose other terminal is connected to an electroluminescent device driving power supply Va. The first switching device Q1 is turned on according to a switching signal SCAN. When the first switching device Q1 is turned on or off, it causes the storage capacitor Cs to be charged or discharged according to a luminance signal DATA. When the discharging voltage from the storage capacitor Cs is applied to the gate terminal, the second switching device Q2 is turned on, thereby causing the electroluminescent device CEL to become luminous by the electroluminescent device driving power supply Va.
When the second switching device Q2 of the electroluminescent device driving circuit shown in FIG. 6 is turned off, the electroluminescent device driving power supply, Va, is applied between the drain and the source of the second switching device Q2. Therefore, it is desirable for Q2 to have a high withstand voltage and low off-current. Accordingly, the semiconductor layer of second switching device Q2 may be made of cadmium selenide (CdSe) or polysilicon (polySi) in order to realize these characteristics.
However, as cadmium selenide degrades with time, the characteristic of drain voltage vs. drain current becomes unstable. Consequently, it is difficult to keep the luminance of the electroluminescent device CEL constant. On the other hand, when polysilicon (polySi) is used, the process temperature for its deposition should be set to a high value. Thus, a large size device cannot be fabricated by depositing the electroluminescent device CEL, which would be degraded by the heat, and the second switching device Q2 on the same substrate.
To solve the aforementioned problems associated with cadmium selenide (CdSe) and polysilicon (polySi), a device with a high withstand voltage may be realized using amorphous silicon, which needs only more moderate process temperature. When such a device with an achievable withstand voltage is used, the device provides characteristics with respect to withstand voltage and off-current which are sufficient for operation as a switching device. However, when the drain voltage is negative, as shown in FIG. 3, drain current is reduced. Therefore, the electroluminescent device driving power supply Va would need to be increased in order to drive the electroluminescent device CEL. Thus, it is impractical to implement the driving circuit shown in FIG. 6 when the semiconductor layer of the second switching device Q2 is made of amorphous silicon.
As shown in FIG. 7, a driving circuit having a dividing capacitor Cdv disposed in parallel with the second switching device Q2 has been proposed. In this circuit, the second switching device Q2 can be designed which requires only a relatively low withstand voltage. However, when amorphous silicon is used for the semiconductor layer, a switching device with a sufficient withstand voltage for the configuration of FIG. 7 has not been achieved. Moreover, when the state of the second switching device Q2 is changed from ON to OFF, a voltage Va equal to the DC component of the electric charge stored in the dividing capacitor Cdv plus to the required voltage VEL of the electroluminescent device is needed for luminescence and will eventually be applied across the drain and source of the second switching device Q2. Consequently, an excessive voltage may be applied across the drain and source of the second switching device Q2 resulting in the electrochemical reaction acceleration factor which reduces the reliability thereof.